Characteristics of Classical Swine Fever Virus Variants Derived from Live Attenuated GPE(−) Vaccine Seed

The GPE(−) strain is a live attenuated vaccine for classical swine fever (CSF) developed in Japan. In the context of increasing attention for the differentiating infected from vaccinated animals (DIVA) concept, the achievement of CSF eradication with the GPE(−) proposes it as a preferable backbone for a recombinant CSF marker vaccine. While its infectious cDNA clone, vGPE(−), is well characterized, 10 amino acid substitutions were recognized in the genome, compared to the original GPE(−) vaccine seed. To clarify the GPE(−) seed availability, this study aimed to generate and characterize a clone possessing the identical amino acid sequence to the GPE(−) seed. The attempt resulted in the loss of the infectious GPE(−) seed clone production due to the impaired replication by an amino acid substitution in the viral polymerase NS5B. Accordingly, replication-competent GPE(−) seed variant clones were produced. Although they were mostly restricted to propagate in the tonsils of pigs, similarly to vGPE(−), their type I interferon-inducing capacity was significantly lower than that of vGPE(−). Taken together, vGPE(−) mainly retains ideal properties for the CSF vaccine, compared with the seed variants, and is probably useful in the development of a CSF marker vaccine.